1. Field of the Invention
The present invention relates to a base material, especially to a base material for an adhesive tape to be used in the process of precision-machining of an optical structural element such as a lens, a semiconductor structural element such as a silicon wafer, or the like. The present invention also relates to an adhesive tape having such a base material.
2. Description of the Related Art
Heretofore, adhesive tapes have been used in the fields of optical industry, semiconductor industry, and so on, for manufacturing structural elements that require precision-machining.
Generally, the adhesive tape for surface protection of a semiconductor wafer can be prepared by the process including the steps of: melting a thermoplastic resin; forming a film-shaped base material as a base sheet by casting the melted thermoplastic resin by the use of a T-die, a blow-extrusion, a calender, or the like; and applying an adhesive agent on at least one surface of the base sheet. The adhesive tape thus obtained is typically used for holding a semiconductor wafer in place during the step of grinding a back side of the wafer and also the steps of dicing and isolating the wafer into pieces.
At first, we will explain the example of using the adhesive tape in the process of grinding a back side of a semiconductor wafer that has a front side on which a predetermined pattern is formed (i.e. the step of back-side grinding).
At the time of grinding the back side of the wafer, the back-side of the wafer is continuously rinsed by running pure water to remove scattered or fragmented remains (i.e., debris) and to reduce heat generated by the grinding. In this case, therefore, an adhesive tape is applied on the front side of the wafer to protect the pattern formed thereon from the grinding.
At the time of performing the steps of dicing and isolating the semiconductor wafer into pieces, an adhesive tape is applied on the back side of the wafer, and that the wafer on which the adhesive tape is being adhered is subjected into each of the steps of dicing, washing, drying, expanding, pick-upping, and mounting, successively.
These steps require that the adhesive tape should have the properties of, for example, having no part of the adhesive agent remaining on a wafer or a chip when the adhesive tape is removed after the step of back-side grinding or the steps of dicing and isolating. For solving this problem, several improvements have been disclosed in patent documents including Japanese Patent Application Laying-open No. 62-153376 (1987) and Japanese Patent Application Publication No. 5-77284 (1993).
The adhesive sheet disclosed in Japanese Patent Application Laying-open No. 62-153376 (1987) comprises an adhesive that contains urethane acrylate oligomer (m.w. 3,000.about.10,000) as a radiation-induced polymerized compound and a base sheet on which the adhesive is applied. Therefore, an adhesive strength of the adhesive sheet is sufficiently decreased by the effect of irradiating it with ultraviolet rays or the like in the step of picking up the semiconductor wafer after dicing. Thus, the adhesive agent does not remain on the back side of the wafer chip. On the other hand, the adhesive tape disclosed in Japanese Patent Application Publication No. 5-77284 (1993) comprises a water-swelling type adhesive agent as an adhesive layer applied on a surface of a base sheet. In this case, therefore, it becomes to possible to remove the adhesive agent attached on the wafer's surface by running water after the grinding step.
Each of the adhesive tapes described above uses a base material which is prepared as a film-shaped material by melting a thermoplastic resin such as vinyl chloride or polypropylene and casting the melted thermoplastic resin by the use of a T-die, a blow-extrusion, a calender, or the like.
In most cases, the base material to be used has a thickness of 50 .mu.m or more in terms of wafer-protection and workability. During the process of forming the base material into a film, however, projections are sparsely formed on a surface of the base material as a result of mixing undesired foreign substances into the material or the existence of insoluble ingredients of its resin component or the like. Each of those projections are in the shape like a fish's eye with a height of about 10 to 50 .mu.m (which is referred as "a fish-eye" in the following description). Therefore, the fish-eye acts as a center of spreading the breakage of a silicon wafer at the time of grinding the back side of the wafer. In this case, furthermore, another problem in that the fish-eye acts as a center of flying a chip off comes up at the time of dicing and isolating the chip.
The generation of fish-eyes may be under control if the base material is of less than 50 .mu.m in thickness. However, such a thin film cannot attain the capability of wafer-protection and leads to poor workability.
For solving these disadvantages, Japanese Patent Application laying-open No. 9-253964 (1997) discloses a base material for an adhesive tape. The base material is formed by casting a liquid radiation-cured resin comprising a mixture of urethane acrylate oligomer and reactive diluted monomer followed by the step of curing the resin by irradiating with a radiation.
However, one of the drawbacks of forming an adhesive tape by directly applying an adhesive layer on a film layer of the base material is to pollute a adherend (e.g., a semiconductor wafer) with a residual adhesive (the so-called "particle") which remains on a surface of the adherend when the adhesive tape is removed from the adherend.
The present inventors and their coworkers have considered that the cause of the particle (i.e., the residual adhesive of the above adhesive agent on the adherend) is a bleeding of unacceptable ingredients from the film layer to the adhesive layer. The unacceptable ingredients have a low molecular weight and are comprised in the radiation-cured resin that forms the film layer of the base material. Eventually, they find that the solution to the above problem lies in using a base material in which a barrier layer is formed on the film layer. The barrier layer is responsible for preventing the bleeding from the film layer to the adhesive layer.